Effectiveness of plasma etching and deposition in electronic device fabrication is reduced by contamination problems. Particulate contamination is a major problem encountered during plasma processing of microelectronic materials. It is estimated that as much as 50% of current semiconductor chip yield loss may be attributed to direct or indirect effects of particulate contamination during fabrication. This fraction is expected to increase as device dimensions are reduced in future technologies. Particles that reduce process yields today range in size from the macroscopic to the sub-micron size.
Particulate contamination also has an extremely deleterious effect on the performance and reliability of microelectronic devices produced by plasma etching or deposition. Particulate contamination can result in device failure, poor film quality, changes in material resistivity, and impurity permeation. Further, as device dimensions are reduced, tighter control of the etching profile requires ever more stringent restrictions on the allowable particle contamination number, density and size. To meet these requirements, tightly controlled, clean rooms are required to avoid particle deposition on produce surfaces during wafer transport and handling.
Improvements in clean room technology and in the handling of in-process substrates (for semiconductor and other applications) have reduced the once appreciable introduction of particles onto substrates during non-process exposure such as wafer handling and transfer. Particulate formation during process steps, including plasma processing, may now contribute a significant fraction of total contamination exposure with corresponding yield reduction. G. S. Selwyn, R. S. Bennett and J. Singh, "In-situ Laser Diagnostic Measurements of Plasma-Generated Particulate Contamination in RF Plasmas" J. Vac. Sci. Tech. A. Vol. 7 (4), pp. 2758-2765 (Jul./Aug. 1989).
In addition, the industry trend is towards "integrated vacuum processing", or "multi-chamber processing". This means that surface contamination previously removed by wet or dry mechanical means will be more complex or impossible to remedy since it now requires removal of the substrate from the vacuum chamber. In multi-chamber tools, particulates which drop onto a wafer before, during, or at the completion or a process step may have an especially severe impact on subsequent process steps in that tool.
Recent studies in our laboratory have shown that certain etching plasmas can produce particulates which may be a significant source of product contamination and device failure. These experiments have shown that particles can be nucleated, grown and suspended in a process plasma until they are significant in size. Sizes of these particles range from the submicrons scale to hundreds of microns in diameter. In addition, when the plasma is initiated, our studies have shown that particles are transported from the wall regions of the chamber into the central plasma region. The problem is that the particles ultimately fall onto devices being fabricated in the same manufacturing environment. If particles fall before or during film deposition or pattern transfer, they can disrupt the process step. If they fall at the end of a process step, the particulates may disrupt subsequent process steps. These contaminants often produce defects which affect the device yield, performance and reliability. Similar results have been observed in deposition type plasma (PECVD Silane), see R. M. Roth, K. G. Spears, G. D. Stein and G. Wang "Spatial Dependence of Particle Light Scattering in an RF Silane Discharge" App. Phys. Lett., 46(3), 253-255 (1985)).
The effects of particulate contamination can be magnified when selective plasma etching processes are used. Certain plasma etching processes rely on a combination of feed gases and etching conditions to selectively etch material surfaces on the wafer. The chemical formation of particulates which are etching at a slow rate in these highly selective plasmas results in micromasking, or an irregular surface often referred to as "grass". This spike or hill of unreacted material will also degrade the device performance and reduce process yield.
Contrary to common belief, presence of these particulates is not always due to material flaking from chamber walls, but may also be due to gas phase processes such as homogeneous nucleation. This suggests that particle contamination problems may not be eliminated solely by rigorous attention to clean room techniques and frequent cleaning of manufacturing equipment. Instead, since the plasma itself can result in product contamination, this problem may pose a "base level" of contamination even with the higher clean room technology. It is therefore important to develop means to operate the plasma while controlling or eliminating particle formation. Further, techniques are also necessary for removing particles, once present in a process.
Laser-light scattering studies on our laboratory have indicated that the plasma composition and gas flow have a pronounced effect on the formation of particle contamination in etching plasmas. In particular, faster gas flow resulting in shorter residence time in the plasma as well as lower pressures and shorter plasma exposure, all tend to inhibit particle formation in certain plasmas. This suggests a mechanism of nucleation and growth for particle production. Feed gas chemistry also has an important effect on particle formation. Chlorine-containing plasmas are highly prone to particle formation, although non-chlorine plasmas, such as CF.sub.4, can produce particles on a smaller scale. Most important, however, has been the discovery by spatially-resolved laser light scattering experiments, that particle growth occurs primarily at the sheath boundary, and may be confined to a vertical region or less than 1 mm thick.
This same region has been shown experimentally to trap plasma negative ions, resulting in high concentration of plasma negative ions in this region as explained by the reference, G. S. Selwyn, L. D. Baston and H. H. Sawin, "Detection of C1 and Chlorine-containing Negative Ions in RF Plasmas by Two-photon Laser-induced Fluorescence" Appl. Phys. Lett., 51 (12), 898-900 (1987). It has been explained theoretically by the reference: M. S. Barnes, T. J. Colter and M. E. Elta, "Large-signal Time-domain Modeling of Low-pressure FR Glow Discharges" J. Appl. Phys., 61(1), 81 (1987).
Applicant's have discovered that by modifying the surface of an electrode on which a workpiece is disposed or by modifying the surface of the workpiece particles in the plasma chamber can be channeled to predetermined locations in the plasma chamber from where they can be removed. This is achieved by a predetermined pattern of protuberances and filled or unfilled grooves in the electrode or wafer surface.
U.S. Pat. No. 4,461,237 to Hinckel et al. describes a plasma reactor for etching and depositing material at an enhanced rate on a semiconductor wafer. The effect is attributed to weakening the electric field outside the regions of the electrode opposite the wafer. The electrode opposite the wafer has holes or apertures which are filled with a dielectric such as quartz. The holes are concentric with respect to the substrates which are being etched or are angular around the electrode. The dielectric insert must be similar in shape to the substrate which is being etched. In contradistinction, the protuberances or grooves which can be filled with the dielectric according to the present invention are not required to be similar in shape with the workpiece substrate. The protuberances or grooves instead provide drainage channels in the equipotential surfaces by which contamination particles can be drained off. The protuberances or grooves of the present invention are generally at the periphery of substrate or between substrates. The Hinckel dielectric insert must be on the electrode opposite the workpiece. In contradistinction, according to the present invention, the dielectric protuberances or dielectric filled groove is generally on the same electrode as the workpiece. The field generated by the Hinckel apparatus is sufficient to intentionally alter the etch or deposition characteristics of the plasma. In contradistinction, according to the present invention, weaker fields are designed not to influence low mass electrons or ions for the purposes affecting the etch or deposition but the much weaker fields are designed to provide a directed radial gradient in the region of the sheath boundary. It is this region in which particles substantially accumulate in plasmas. This gradient affects the motion of massive particles, but has a substantially lower effect on much lighter ion and electrons.
It is an object of this invention to alter the electrostatic potential within a process chamber to selectively collect particles in predetermined regions for subsequently draining them away from the workpiece or out of the processing chamber.
It is an object of this invention to selective alter the electrostatic potential by providing grooves or protuberances or combinations thereof on the surface of the electrode holding a workpiece to be etched or upon which material is to be deposited or from which the material is to be removed or by providing protuberances or grooves on the workpiece itself.